2007 CEDAR ST 2023-04-10 - Laserfiche WebLink

DESIGN AND CONSTRUCTION STANDARDS AND SPECIFICATIONS 6-10.1. STORMWATER REQUIREMENTS FOR DEVELOPMENT IN THE COMBINED SEWER AREA • . _Developmentpmjects.draining..to.the:combined sewer system are not required to meet the same stormwater management standards as projects which drain to a separated stormwater system. Instead,projects must comply with the following requirements for design of stormwater systems associated with development. Public Infrastructure development/redevelopment(such as road and utility construction)within the Public Right of Way is exempt from these requirements.Projects need not meet the standards included in.Section 4 of this.DCSS or the adopted Stormwater Management Drianual,although the following section draws from • portions of both:Unless•otherwise•stated below-stormwater facilities constructed to meet the combined sewer area requirements should be constructed and maintained in accordance with the requirements of Section 4 and the Stormwater Management Manual. 6-10.1(1) FLOW CONTROL • Projects shall employ Flow Control BMPs in accordance with the following thresholds, standards,.and requirements to reduce the impacts of stormwater runoff from hard surfaces and land cover.conversions. Thresholds • • The following projects require construction Of From/Control BMPs.to achieve the Flow Control Performance Standard.If a project meets any of the following thresholds,Flow Control BMPs are required.The project proponent•must•demonstrate that the projeet•does•not meet-any-of the following thresholds for Flow Control . • BMPs to not be required. • Projects that have a total of 200 square feet or more of netnew impervious surface,or • Projects that convert 1/4 acres or more of native vegetation,pasture,scrub/shrub,or unmaintained non-native vegetation to lawn or landscape. Flow Control Performance Standard The combined sewer collection system is designed to a goal of conveying the 25-year recurrence storm . event without flooding. That analysis was based on the land cover:in place at the time that the conveyance -system-was constructed: Any increases in-runoff rates have the potential to cause or exacerbate flooding in the system.Therefore,projects which exceed the threshold for flow control shall comply with the following standard: • • Stormwater discharges shall match developed discharge durations to existing site durations for • the-range•of existing-discharge'rates=from•the-10-year peak flow through the full 100-year peak . ,flow. • Peak runoff rates from the developed site shall not exceed existing site runoff rates in the 25-year and 100-year return intervals. The existing condition to be matched shall be the site as it exists at the time of application. Calculations shall be performed using a continuous simulation modeling program approved by the Washington • • .SANITA►RY,SEWERS 6-1i •